1. Field of the Invention
This invention relates in general to the construction of implants and, in particular, to a new and useful implant alloy and to the use of an improved alloy for body implants.
2. Description of the Prior Art
Implants for surgery are made from stainless steel (standards: USA ASTM F55, ASTM F 56; GB Brit. Standard 3531-1962), cobalt alloys under the trade names Vitallium and others (standards: USA ASTM F54; British Standard 3531-1962), titanium (standards: USA ASTM F67-66; British Standard 3531-1962/1967) and a Ti 6Al 4V-alloy (standard for clinical tests in USA: ASTM 1968). The second material is a cast alloy; of such cast materials it is known that porosities may occur and without particular techniques, such alloy has a very coarse grain structure. However, for surgical implants, these are undesired properties, since coarse grain reduces the strength of the alloy and controlling freedom from pores in the alloy is expensive.
All the metals show sufficient tissue compatibility.
The following Table 1 gives information about the mechanical properties of the named materials for implants.
TABLE 1 ______________________________________ tensile fatigue elasticity strength elongation strength modulus Material Kg/mm.sup.2 % kg/mm.sup.2 kg/mm.sup.2 ______________________________________ Stainless steel annealed 60 40 30 19000 Stainless steel cold formed 90-100 15 40 19000 Technically pure Titanium 55 40 28 11000 Vitallium, cast and annealed 55 10 28 20000 ______________________________________
The view that implants of stronger dimensioning are necessary for operative fracture treatment is only valid as regards strength, which is necessary to prevent fatigue-conditioned breakage of the implant. The strong dimensioning, however, also makes implants mechanically stiff. Implants which are too rigid, however, do not permit functional loading of the bone bridged by the implant, and there results dangerous weakening of the bone substance or decalcification and further fractures. The ratio of tensile strength to elasticity modulus for stainless steel is 100/20000 and for titanium is 55/11000, which is therefore about as much. The ratio is low, and the strength of a biomechanically favorably dimensioned implant, i.e., of adapted rigidity, is insufficient to eliminate complications because of implant breakage. The tensile strength should preferably amount to at least 100 kg/mm.sup.2, and the fatigue strength should amount to about 60 kg/mm.sup.2.
A further requirement for implant materials is full tissue compatability. Ferguson, et al. (Journal of Bone and Joint Surgery 41A, 737, 1959; id. 42A, 77, 1960; 44A 323, 1962; Journal of Biomedical Materials Research 1, 135, 1967) in experiments on the rabbit after implantation over some months, have measured the concentration of the metal lying in the tissue, and they have characterized reactions of this tissue. The results are shown in the following Table 2.
TABLE 2 ______________________________________ metal includes enrichment in toxic reaction of Material the tissue elements the tissue ______________________________________ Stainless steel 1,7 ppm Ni connective tissue up to ca. 100.mu.m Vitallium 1,6 ppm Co connective tissue up to ca. 100.mu.m Titanium 2,6 ppm none connective tissue under ca. 15.mu.m ______________________________________
The strong membrane of connective tissue around stainless steel and cobalt alloys is a rejection reaction of the body. On the other hand, the toxicity examinations of Hulliger, et al. (Zeitschrift fur die gesamte experimentelle Medizin 144, 145, 1967) show growth inhibition on connective tissue cultures in the presence of metals and their chlorides for Cu, Co, Ni, Sb, Sn, Zn, etc. These are the toxic reactions which lead to encapsulation and inflammations and therefore restrict the tissue vitality.
Electrochemical methods alone are suitable for measuring the extraordinarily small corrosion of resistant metals in the body. For the "in vitro"-experiment, electrolytes of similar composition as body liquid are needed. "In vivo"-experiments are used because only then the Redox process operates correctly, and also direct effects of the adjacent, displaced tissue are determinable. In such experiments, the test metal with an insulated contact wire and a flexible tubing, used as salt bridge arriving at the surface of the test metal and connected to a reference electrode, is introduced in the experimental animal (rabbit) and for the measurement, the salt bridge and the conductor to the metal are connected to the measurement apparatus, while the auxiliary electrode is put onto the skin. The so-called polarization resistance is measured, which may be converted into the corrosion current and corrosion rate. After 3 to 8 weeks, during which the corrosion is repeatedly determined, the animal is sacrificed and tissue lying on the implanted metal is examined histologically.
Each metal, even Pt, corrodes in the body liquid and results in corrosion products (hydroxides, oxides in different oxidation states, chlorides, sulfides, etc.). These corrosion products arrive through mechanical transport (phagocytes, etc.) and chemical transport (solution, dissociation and reprecipitation) into the tissues and the circulatory system. The corrosion is always stronger than the ability of the body to transport away all corrosion products; tissue reactions are the result (connective tissue is formed, necrosis, inflammations). These reactions are on the basis of the histological examination classified in three groups:
1. "Sterile abscess", in which the tissue lying on the metal is necrotic and round cells exist, and the connective tissue is pervaded only within a moderate distance with blood vessels and capillaries; PA1 2. Encapsulation of the foreign body through more or less thick connective tissue, but lacking inflammationary reaction; and PA1 3. Filling up reaction through loose connective tissue, without encapsulation and vascularization up to the metal, therefore full vitality.
In the following Table 3, several results of corrosion experiments "in vivo" in rabbits and hisological results are put together. Further, they contain element data. The metals with an electrical lead and salt-bridge to reference electrode implanted and measured periodically by polarization resistance technique.
TABLE 3 __________________________________________________________________________ corrosion current corrosion rate Material A/cm.sup.2 g/cm.sup.2 tissue reaction __________________________________________________________________________ Nickel 1 .sup.. 10.sup.-7 4 sterile abscess (animal after 2-3 weeks dead) Iron 1 .sup.. 10.sup.-6 20 Aluminum 4 .sup.. 10.sup.-7 2 encapsulation with- out inflammation Stainless steel 1 .sup.. 10.sup.-9 0,1 Titanium 7 .sup.. 10.sup.-10 0,007 Zirconium 7 .sup.. 10.sup.-10 0,012 loose, thin connect- ive tissue, vascular- ized; full vitality Platinum 1 .sup.. 10.sup.-8 0,3 Tantalum 5 .sup.. 10.sup.-9 0,12 Niobium 2 .sup.. 10.sup.-9 0,03 __________________________________________________________________________
Results concerning tissue reaction are obtained from rabbit tests at the same time as the corrosion measurement (histology after sacrificing the animal) and from separate tests of implanted metals in rabbits and rats. D. C. Mears, "Journal of Bone and Joint Surgery", Vol. 48B, p. 567 (1966) suggests titanium-niobium, titanium-tantalum alloys (page 575, line 8) ". . . be discarded in favor of more inert materials - such as cobalt, chromium alloys, pre titanium . . . ". The sense of the word "inert" is here "chemically inert" and, in relation to low corrosion rate and has to be distinguished from the notion "non-toxic". In fact, Mears mixes in the same phrase also alloys with cobalt which is explicitly a toxic element.
In group 1 in particular, the metals Cu, Co, Ni, V and Sn are included, and in group 2, fall stainless steels, vitallium, Ag, Au, Al, Fe, Cr, and Mo. It is apparent that for groups 1 and 2, a strong tissue reaction is not unconditionally interrelated with strong corrosion. Al and Fe corrode strongly like Ni, but have a harmless reaction. Group 3 comprises the five last-mentioned elements of Table 3.